1. Field of the Invention
The present invention relates to a method and a device for driving light shutters, and more particularly to a method and a device for driving a light shutter comprising electrooptical elements of PLZT or the like.
2. Description of the Related Art
U.S. Pat. No. 4,902,111 and U.S. Pat. No. 4,932,761 disclose light shutters wherein PLZT is used as a substance having an electrooptical effect. These PLZT light shutters have a high response speed and are therefore useful as optical heads for electrophotographic printers.
These PLZT light shutters have the drawback that the characteristics thereof vary with variations in temperature and with the lapse of time.
We determined the characteristics of the PLZT light shutter using the arrangement shown in FIG. 1. With reference to FIG. 1, a PLZT element 3 having a pair of electrodes 4, 5 is interposed between a polarizer 1 and an analyzer 2. The polarizer 1 and the analyzer 2 are so arranged that the polarization angles of light passing therethrough are orthogonal to each other. When an electric field is applied to the PLZT element 3, the polarization angle of light passing through the polarizer 1 is rotated through 90.degree., so that the light passes through the analyzer 2. On the other hand, if no electric field is applied to the PLZT element 3, the polarization angle of the light through the polarizer 1 is not rotated, so that the light is blocked by the analyzer 2. With this arrangement, a drive voltage is applied to the electrode 5 with the electrode 4 grounded to subject the PLZT element 3 to the electric field. The light from the analyzer 3 is detected by a photodetector 6, and the output from the photodetector 6 is amplified by an amplifier 7.
FIGS. 2, 3 and 4 show the characteristics of the PLZT element 3 determined by the above arrangement.
First, FIG. 2 shows the relationship between the drive voltage for the PLZT element 3 and the amount of transmitted light when the ambient temperature is 25.degree. C., 50.degree. C. and 70.degree. C. The graph shows that the drive voltage (half-wavelength voltage) for giving a maximum amount of transmitted light increases as the ambient temperature rises. Acccordingly, even in the case where a half-wavelength voltage VH at room temperature (25.degree. C.) is used as the drive voltage, the light amount decreases when the ambient temperature rises.
Next, FIG. 3 shows the relationship between the drive voltage for the PLZT element 3 and the amount of transmitted light in the initial state (solid line) and after driving for a long period of time (broken line). The diagram shows that in the initial state, the absolute value of half-wavelength voltage VH for giving the maximum amount of light is the same irrespective whether the polarity of the drive voltage is positive or negative. Nevertheless, after a positive drive voltage has been applied to the PLZT element 3 with irradiation with light for the long period of time, the drive voltage-light amount curve shifts toward the positive side in its entirety. With the drive voltage set to VH, therefore, the amount of transmitted light decreases with the lapse of time. For example, FIG. 4 shows variations in ON/OFF ratio (the ratio of ON-state to OFF-state in the amount of transmitted light) in the case where the PLZT element 3 is continuously driven while the element is being irradiated with light using a 150-W halogen lamp as the light source, a drive pulse voltage of 50 V (duty ratio: 50%) and a drive frequency of 2 kHz. FIG. 4 reveals that the ON/OFF ratio decreases with time from about 50 in the initial state to about 20. The decrease in the ON/OFF ratio is considered to be attributable to a space-charge layer formed in the vicinity of the electrode by carriers excited within the PLZT element 3 when the element 3 is driven and irradiated with light. The space-charge layer sets up within the PLZT element 3 an electric field which causes a leak of light when the element is turned off and a reduction in the amount of light when the element is turned on.
When the PLZT light shutter is used as the optical head for electrophotographic printers, the above-mentioned variations in the amount of transmitted light alters the amount of light to which the photosensitive member is to be exposed, consequently making it impossible to obtain images of high quality.